The present disclosure generally relates to the removal of particulates and other contaminants from flue gas produced during combustion using a dry scrubber flue gas desulfurization system during normal operation. In particular, this disclosure relates to new and useful methods and systems for capturing sulfur dioxide (SO2), sulfur trioxide (SO3), HCl, and other acid gases by injecting dry sorbent into a gas stream and passing the gas stream through a spray dryer absorber to disperse the sorbent in a baghouse during the use of a pollutant-forming fossil fuel in a combustion system.
During combustion, the chemical energy in a fuel is converted to thermal heat, which can be used in various forms for different applications. The fuels used in the combustion process can include a wide range of solid, liquid, and gaseous substances, including coal, oil (diesel, No. 2, Bunker C or No. 6), natural gas, wood, tires, biomass, etc.
Combustion transforms the fuel into a large number of chemical compounds. Water (H2O) and carbon dioxide (CO2) are the primary products of complete combustion. However, other combustion reactions with chemical components in the fuel result in undesirable byproducts. Depending on the fuel used, such byproducts may include particulates (e.g. fly ash), acid gases such as sulfur oxides (SOx) or nitric oxides (NOx), metals such as mercury or arsenic, carbon monoxide (CO), and hydrocarbons (HC). The emissions levels of many of these byproducts are regulated by governmental entities, such as the U.S. Environmental Protection Agency (EPA).
Several different technologies exist for removing such byproducts from the flue gas. In one method, known as spray drying chemical absorption or dry scrubbing, an aqueous alkaline solution or slurry, which has been finely atomized, is sprayed into the hot flue gas downstream of the combustion chamber in which the fuel was combusted. The alkaline reagent reacts with the pollutants, and particulates are formed. The water evaporates and cools the hot flue gas. The exiting cleaned flue gas typically has a moisture content of about 10% to about 15%. The flue gas then travels to a particulate collection device, generally a baghouse, where the particulates are removed from the flue gas, which is then sent to a stack.
When a combustion system, such as a boiler having a furnace, is started up from cold conditions such as ambient temperatures, the furnace usually burns natural gas or diesel (No. 2) oil to “warm up” the boiler before switching over to coal. A furnace temperature of about 400° F. to about 500° F. is needed before coal can start to be burned. Due to various startup conditions and safety requirements, the furnace can be started and stopped several times before attaining steady-state operations. Complete startup can take anywhere from 8 hours to up to 2 days to complete, depending on the problems encountered.
The dry scrubbing desulfurization process does not work well at low temperatures. In particular, the temperature of the flue gas typically needs to be at least 220° F. to use the spray dryer absorber, so that the water can be completely evaporated. During startup, the temperature of the flue gas that passes to the spray dryer absorber may be below this threshold temperature, yet SOx and other pollutants are still being produced. In addition, the furnace generally reaches the coal operating temperature of 400° F. to 500° F. before the flue gas attains a temperature of 220° F. in the spray dryer absorber. This results in higher SOx emissions during startup. In addition, the baghouse generally requires 30 to 60 minutes of operation after the spray dryer absorber has started to accumulate significant alkaline material and achieve significant SO2 removal.
Previously, emissions regulations did not cover “upset” periods such as startup, shutdown, and malfunction. However, it would be desirable to reduce such emissions due to increasing regulatory restrictions. Methods that can reduce such emissions during startup would be very helpful.